Mr Baden-Clay has been found guilty of murdering his wife Allison at their home in the affluent western Brisbane suburb of Brookfield on April 19, 2012, and dumping her body on the muddy banks of Kholo Creek, 14 kilometres away.

Allison and Gerard Baden-Clay, with their three children. Photo: Supplied

Dr Robertson reviewed the results of a post-mortem examination conducted on Allison Baden-Clay's body and suggested the mother-of-three may have suffered “Serotonin Syndrome”, causing her to hallucinate on the night she was last seen alive.

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He said excessive levels of the antidepressant drug Sertraline, sold as Zoloft, could cause Serotonin Syndrome, prompting unusual behaviour and profound confusion.

Dr Robertson was among the final witnesses to be called to testify.

The defence seized upon his testimony, suggesting Mrs Baden-Clay took her own life in the early hours of April 20, 2012, while under the adverse effects of Zoloft.

It also suggested Mrs Baden-Clay may have died as a result of misadventure or accident, brought about by medication.

However, the jury never heard that Dr Robertson was named as an “unindicted co-conspirator” in the 2000 murder of Gregory de Villers in San Diego, California.

Mr de Villers’ wife Kristin Rossum was convicted in 2002 of murdering her husband and attempting to make his death look like a suicide.

Dr Robertson was embroiled in an affair with Ms Rossum. He was the head of the toxicology laboratory at the San Diego County Medical Examiner’s Office, where she worked as a junior toxicologist.

Prosecutors in Rossum’s trial alleged Dr Robertson helped his then-lover steal the highly-toxic drug fentanyl from the laboratory, which she administered to her husband to kill him.

Evidence included passionate emails exchanged between Dr Robertson and Rossum, as well as computer files and professional writings revealing Dr Robertson’s specialist knowledge of fentanyl.

Rossum maintained her husband took his own life after learning of her affair and fearing their marriage would end.

The case was dubbed the “American Beauty murder” because Mr de Villers' chest was covered in red rose petals, reminiscent of a scene in the 1999 movie.

Prosecutor Dan Goldstein said Dr Robertson was "inextricably wound up in the case" and protected Rossum "in every way".

But Dr Robertson has never been charged.

Dr Robertson returned to Australia before Rossum’s trial, because his job-related visa expired. He did not testify for his ex-lover and declined to take a lie detector test.

He too was married when he and Rossum began their affair in the spring of 2000, two months after he began working in San Diego and less than a year after she married Mr de Villers.

He has since divorced, remarried and relocated with his family to Brisbane, where he works as an independent forensic consultant for hire, compiling toxicology reports for legal practitioners for $800 to $1500.

When contacted by Fairfax Media, the toxicologist said he had never received a summons from US authorities alleging his involvement in the crime, but he declined to comment further.

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